There is a considerable call for a two-armed system for unit cranes, for instance. Such a crane consists of a rigid arm and thereafter an arm composed of a number of part-arms joined together which can be influenced hydraulically in such a manner that the part-arms form a polygon. This arm is in turn attached to a stand or to said stand via a supporting arm. The two arms in the system, together with a supporting arm if used, can thus be arranged to be situated one above the other and also so that all the arms are extended. Various units can be attached at the tip of the rigid arm, for use in clearing trees and bushes. Units functioning as excavators or loading means can also be attached, or even a concrete pump or concrete hose. In existing systems with two arms, the arm able to form a polygon is influenced by a hydraulic unit arranged between the arm parts so that, when the polygon is extended, the unit is located substantially in the middle of the polygon. Certain problems have been found to occur when operating such two-armed systems, as regards speed control of the top of the rigid arm, and problems also occur as regards the force exerted at said tip. Such a unit crane is suitable fitted on a caterpillar vehicle and this involves the problem of how to fold up the crane when it is not in operation. The arms will usually have a substantially vertical position, or they will be situated on the roof of the vehicle. There is also the problem of the actual joints of the unit arms being as stable as possible.